Food, Fasting, and Physiology: how far studies have been carried out, especially on energy and water demand in our body

Glycogen lasts for only so long, and before these reserves run out the body begins to use a more complex, energy-rich source called lipids (fat). The person who fasts, drinking only water, will deplete his glycogen reserves in a timeframe ranging from hours to a day or two (depending on energy demands) and will then be in fat respiration (breakdown).